Dispersion of magnetic nanoparticles in a nematic liquid crystal host: Phase diagram, Fredericks transition and deformation of droplets

During the seventies when the main properties of magnetic fluids were first understood, (superparamagnetism, birefringence, hydrodynamics instabilities...) theoreticians [1] have imagined the possible advantages of a ferrofluid with a thermotropic nematic liquid crystal (LC) as a solvent. Such a “ferronematic” would indeed combine the properties of two systems which become optically anisotropic (birefringent) under electrical and magnetic fields. The today widely used liquid crystals displays (LCDs) are based on the transition between transparent and opaque state of LCs, controlled by electric fields. For certain applications, magnetic fields could be used instead if we could lower down the threshold magnetic field intensity Hc of the so called Fredericks transition arising from the competition between alignment of LC molecules by surfaces and by an applied magnetic field. This idea motivated our experimental study of dispersion of nanoparticles made of maghemite iron oxide (Γ-Fe2O3) and 5-CB, one of the most standard nematic LCs which is convenient due to its nematic-isotropic temperature (TN-I=35°C) slightly above room temperature. However, we found that a true (monophasic) ferrofluid with 5-CB as solvent can be obtained only in the isotropic phase (above TN-I), whereas in the nematic state, the system separates between two phases: one the one hand magnetic microdroplets made of a high concentration of magnetic nanoparticles (about 18 vol% from SAXS measurements) in isotropic 5-CB and on the other hand a non magnetic 5-CB nematic matrix [2]. This phenomenon was explained by the thermodynamic laws for a ternary system (nanoparticles – LC – surfactant). Two aspect of these highly magnetic droplets in a LC host matrix where studied : i) their influence on the threshold field Hc of the Fredericks transition of a 5-CB layer sandwitched between two plates with homeotropic alignment conditions; ii) their strong ellipsoidal deformation under a magnetic field of low intensity, which – by analogy with ferrofluid droplets in a non magnetic liquid – provides an experimental measurement of the interfacial tension and tentatively of the anchoring energy of LC molecules onto nanoparticles [3]. ___________________________________________________ [1] F. Brochard, P. G. de Gennes, J. Phys. (Paris), 1970, 31, 691. [2] C. Da Cruz, O. Sandre, V. Cabuil, Journal of Phyical Chemistry B (2005) 109, 14292. [3] J. Deseigne, report of ESPCI engineering school short training period (March 2006)

Magnetic tubules

International audienceDispersion of tubules made of diacetylenic phospholipids (DC8,9PC), in aqueous colloidal dispersions of magnetic nanoparticles is studied as a function of the sign of particles surface charges. In every case the tubule-vesicle transition temperature is decreased by the presence of the magnetic nanoparticles. Electrophoresis experiments on the tubules in pure water permits to conclude on a negative apparent surface charge. We study the magnetic response of the system to a static or stationary rotating field and to a magnetic field gradient. These experiments reveal an excess or a lack of magnetic permeability between tubules and the surrounding medium. Electron microscopy confirms these results showing an electrostatic interaction between the phospholipidic bilayer and the magnetic particles

Oblate-Prolate Transition of Ellipsoidal Giant Magnetoliposomes: Experiments Showing an Anisotropic Spontaneous Curvature (Chapter 11)

International audienceThis chapter contains sections titled:IntroductionLiposome PreparationObservationsAnalysis of Deformations Under a Magnetic FieldInterpretatio

New ferrofluid composites: Magnetism and optics experiments

International audienc

Synthesis of high molar masses of poly(n-butyl methacrylate)-b-polystyrene diblock copolymers by ATRP. Formation of lamellar phases in thin films

International audienceDiblock copolymers are known to self-organize in ordered domains. The morphology of this microphase separation depends essentially on the relative amount of each immiscible block in the copolymer (e.g. spherical, cylindrical, gyroidal and lamellar phases). The lamellar nanoscopic structures can serve as templates for the selective incorporation of inorganic nanoparticles , , for instance to get nanopatterned surfaces or for new applications such as wave guides. , In most of the studies concerning diblock copolymer self-assembly in lamellar periodic structures, diblock copolymers were synthesized by anionic polymerization which is a suitable technique to prepare well-defined high molar mass diblock copolymers. , In the present study, our aim is the synthesis of high molar mass poly(n-butyl methacrylate)-b-polystyrene diblock copolymers by Atom Transfer Radical Polymerization (ATRP). Compared to anionic polymerization, controlled radical polymerization (CRP) is a more versatile technique which requires less drastic experimental conditions. Our results highlight the self-assembly of the symmetric poly(n-butyl methacrylate)-b-poly(styrene) diblock copolymer synthesized by ATRP into a lamellar ordered structure after an annealing treatment

Designing magnetic composite materials from aqueous magnetic fluids

International audienceIn this paper, we report on how to take advantage of good knowledge of both the chemistry and the stability of an aqueous magnetic colloidal suspension to realize different magnetic composites. The osmotic pressure of the magnetic nanoparticles is set prior to the realization of the composite to a given value specially designed for the purpose for each hybrid material: magnetic particles in polymer networks, particles as probes for studying the structure of clay suspensions and shape modification of giant liposomes.First, we show that the introduction of magnetic particles in polyacrylamide gels enhances their Young modulus and reduces the swelling caused by water. The particles cause both a mechanical and an osmotic effect. The latter is strongly dependent on the ionic strength and is attributed to an attraction between particles and the polymeric matrix.In the second part, we determine the microscopic structure of suspensions of laponite as a function of concentration, by combining SANS and magneto-optical experiments with the probes. This study requires conditions suitable for including the magnetic particles as probes without disturbing the clay suspensions.The third part presents giant magnetoliposomes, which encapsulate magnetic nanoparticles. Shape transitions are obtained with either a magnetic field or an osmotic stress

Magnetic nano- and microparticles for metal removal and environmental applications: a review

International audienceA review. Besides the numerous applications of magnetic particles in the fields of medicine, diagnostics, mol. biol. and bioinorg. chem., a high potential exists for these particles in environmental sciences. Several methods have been proposed these last years for the sepn. of metals from wastewater using either micro- or nano-magnetic particles. We describe here the synthesis of such particles, compare their magnetic properties, and discuss the possibility of selectivity for metals ions, namely radionuclides